Worldwide there has been an alarming increase in the incidence of skin cancer over the past two decades, particularly in the populated Sunbelt regions. This near epidemic of skin cancer has resulted in many organizations, including the National Skin Cancer Foundation, heightening public awareness of the problem of solar radiation and encouraging preventative measures. One of the most notable preventative changes has been the development of the sun protection factor (SPF) which provides rating for sun protection given to various sunscreen products. The SPF system for sunscreens is based on units of time required for a given skin type to reach erythema condition under selected radiation exposure and ranges from 0 to 50 with increasing protection.
Other than the skin, the only other human organ directly exposed to sunlight is the eye. It is well-known that sunlight, or solar radiation, at the earth's surface can cause damage to the eye. Although other parts of our body, including the immune system, may be adversely affected by sunlight, damage to the eye from certain wavelengths of sunlight is well documented. Examples of such damage include eyelid cancer, cataract, pterygium or pinguecula, keratitis (snow blindness) and possibly macular degeneration. Since virtually all traditional sunscreens are toxic to the cornea and would interfere with vision, the typical method of sun protection for the eye, beyond normal anatomical and physiological protection, has been the use of sunglasses.
The spectrum of solar radiation incident on the earth's surface extends from 290 nm to 23 um. The range of solar radiation which affects the human eye is from 280 nm to 1400 nm. Radiation below 280 nm is nearly completely absorbed in the earth's stratosphere and does not reach the ground. Radiation above 1400 nm does not transmit through the human eye. The spectrum of radiation, or wave band, with wavelengths between 280 nm and 1400 nm is that which pertains to protecting the eye from natural sunlight exposure.
The eye contains many different structures that absorb different wavelengths of radiation in this range. The eyelids, conjunctiva, cornea, sclera, choroid, aqueous humor, vitreous and retina all absorb and are sensitive to different wavelengths. For instance, the cornea is sensitive to radiation approximately between 280 nm and 315 nm. The retina is sensitive to radiation approximately between 315 nm and 515 nm and also approximately between 700 nm and 1400 nm.
While some structures of the eye are more sensitive to certain wavelengths than other structures of the eye are, radiation still must pass through some structures in order to reach others. Some structures thus act similar to a filter. Wavelengths between approximately 400 and 1400 nm are transmitted by the ocular lens to the retina. The ocular lens of the human eye absorbs most of the wavelengths below 400 nm. Consequently, the ocular lens provides the primary protection for the retina from the hazardous effects of short-wavelength radiation.
The damaging effect of solar radiation upon the eye is dependent upon the wavelength or energy content of the photons. Long-wavelength radiation, in the near infrared, is relatively harmless, whereas shorter wavelength radiation, in the near ultraviolet, is very damaging. For example, there is approximately 3000 times more energy required at 1064 nm than at 350 nm to produce a retinal lesion of equal severity. Nevertheless, radiation between 380 nm and 1400 nm is damaging to sensitive structures within the eye.
Although visible light is defined by a narrow band of spectral radiation, the eye absorbs radiation beyond that range and can be damaged by such radiation. Infrared radiation absorbed by the eye has wavelengths between 700 nm and 1400 nm, even though none of this is visible to the human eye. Within the visible range (380-700 nm), the damaging, or toxic effects, of radiation increase progressively as photon energy rises, but not in a simple, linear manner. However, there is a sudden rise in the amount of damage produced in the retina when the photon energy reaches a wavelength of approximately 510 nm. This is followed by a precipitous increase in the severity of deleterious effects through the remainder of the visible part of the spectrum and continuing into the ultraviolet. The high-energy segment of the visible region (approximately 380 to 515 nm) is more hazardous to the retina than the low-energy portion (approximately 515 to 700 nm). Moreover, because this increased toxicity occurs at the border between the perceived colors of green and blue, the phenomenon is referred to as the blue-light hazard. Blue-light radiation is typically associated with wavelengths approximately between 380 nm and 515 nm.
Ultraviolet (UV) radiation comprises invisible high-energy rays from the sun that lie just beyond the violet/blue end of the visible spectrum. Although more than 99% of UV radiation is absorbed by the lens of the eye, a portion reaches the light-sensitive retina. The UV radiation present in sunlight is not useful for vision. There are good scientific studies that support that UV absorption by the eye contributes to age-related changes in the eye and a number of serious eye diseases.
Ultraviolet radiation in sunlight is commonly divided into three components: UV-A (380 to 315 nm) radiation that causes tanning but is also thought to contribute to aging of the skin and skin cancer; UV-B (315 to 280 nm) radiation that can cause sunburn and predispose to skin cancer; and, UV-C (280 to 110 nm) radiation that is nearly completely absorbed by the ozone layer before reaching the Earth's surface. UV radiation plays a role in the development of various ocular disorders including age-related cataract, pterygium, cancer of the skin around the eye, photokeratitis and corneal degenerative changes, and may contribute to age-related macular degeneration.
Clinical experience, evidence from accidental exposures, and other experimental studies show that UV-B is more damaging to the eye, presumably because it has higher energy than UV-A. The cornea and lens of the eye absorb most of the UV-B; therefore it can cause damage to these tissues but will not normally damage the retina. However, the retina, if exposed to UV-B radiation, can be damaged. UV-A radiation has lower energy than UV-B and penetrates much deeper into the eye to cause injury to the retina and lens. Neither UV-B nor UV-A has been shown to be beneficial to the eye.
Cataracts are a major cause of visual impairment and blindness worldwide. Cataracts are a cloudiness of the lens inside the eye that occurs over a period of many years. Laboratory studies have implicated UV radiation as a causal factor for cataract. Furthermore, epidemiological studies have shown that certain types of cataracts are associated with a history of increased UV radiation exposure.
Age-related macular degeneration is the major cause of reduced vision in the United States for people over age 55. Exposure to UV and intense violet/blue visible radiation is damaging to retinal tissue in laboratory experiments; thus scientists have speculated that chronic UV or intense violet/blue light exposure may contribute to the aging processes in the retina.
Pterygium is a growth of tissue on the conjunctiva of the eye that may extend onto the clear cornea where it can block vision. It is seen most commonly in people who work outdoors in the sun and wind, and its prevalence is related to the amount of UV exposure. It can be removed surgically, but often recurs, and can cause cosmetic concerns and vision loss if untreated.
Photokerititis is essentially reversible sunburn of the cornea resulting from excessive UV-B exposure. It occurs when someone spends long hours on the beach or in the snow without eye protection. It can be extremely painful for 1-2 days and can result in temporary loss of vision. There is some indication that long-term exposure to UV-B can result in corneal degenerative changes.
Children are not immune to the risk of ocular damage from UV radiation. They typically spend more time outdoors in the sunlight than adults do. Also, in young children, transmittance to the retina is greater because much less light is absorbed by the lens. Solar radiation damage to the eye appears to be cumulative and increases the risk of developing an ocular disorder later in life. Therefore, it is prudent to protect the eyes of children against UV radiation by wearing a brimmed hat or cap and sunglasses.
Sunglasses are typically the most practical method of protecting the eyes from solar radiation. Both the lens and frame absorb and prevent radiation from reaching the eye. However, consumers typically purchase sunglasses for fashion or comfort reasons rather than protective ones. And while there is rising public awareness of the benefits of sunscreen and the meaning of sunscreen SPF factors, there is not a similarly standardized and accepted rating system for sunglasses. There are currently no labeling requirements for sunglasses by the American National Standards Institute. Manufacturers can voluntarily indicate whether the sunglasses are intended for normal use or prolonged sun exposure and whether the sunglass lenses are designed for special purpose, dark, general purpose, or cosmetic tint. However, there are no metric-based requirements for indicating the percentage of radiation the lens absorbs or what wavelengths of radiation are absorbed. There are also no requirements for indicating the amount or spectral location of wavelengths absorbed by the sunglass frame or about how much coverage the frame provide.
On Sep. 7, 1999, a United States patent issued for a protective rating system for eyewear to Hall, whereby potentially three factors are measured relating to a sunglass product. The first factor signified the amount of UV radiation that reached the eye through the sunglass lens. The second factor signified the amount of blue light (380 nm to 515 nm) transmitted through the sunglass lens. The third factor represented the frame coverage and signified the amount of incident light reaching the eye from around the sunglass lens and frame, including scattered and reflected light. The three factors, or a combination of them, are presented for informational purposes to the consumer.